The present inventive concept described herein relates to a semiconductor memory device, and more particularly, to a semiconductor memory device having a command storing function, which is capable of storing operation environment information when the semiconductor memory device is powered on.
In recent years, electronic appliances have been designed as necessarily including semiconductor devices such as processors and memories, and other semiconductor devices. In data processing systems for example, processors utilize main memory such as dynamic random-access memory (DRAM) or magneto resistive random-access memory (MRAM) as working memory. DRAMs or MRAMs may be mounted on a module substrate to constitute a memory module. A memory controller may typically control the memory module based on a request of a processor.
DRAM is volatile memory and may include a plurality of memory cells, each of which includes one access transistor and one storage capacitor. In contrast, spin transfer torque magneto resistive random access memory (STT-MRAM) is nonvolatile memory and has memory cells each including a magnetic tunnel junction (MTJ) device and a selection transistor. Data stored in the memory cells of STT-MRAM are not lost even when a power error occurs or power is shut off.
The MTJ device may include a fixed layer, a free layer and a tunnel layer interposed between the fixed and free layers. A magnetization direction of the fixed layer is fixed, and a magnetization direction of the free layer is the same or opposite the magnetization direction of the fixed layer based on conditions.
When an arbitrary one among memory cells does not maintain its stored data over time, the memory cell becomes a defective memory cell which may cause a read error. As memory cells become defective, internal circuits configured to perform functions according to data stored in DRAMs or MRAMs including the defective memory cells may in turn malfunction and/or become defective over time.